FreeBSD has mono

C♯ and mono are both a blessing and a curse. Most documentation, FreeBSD-specific or not, on the internet are more or less outdated. Maybe I was looking in the wrong places. It was really frustrating. Soon this will be outdated too.

Start by adding lang/mono. Then add x11-toolkits/libgdiplus.

If you want to use the Gtk libraries, add x11-toolkits/gtk-sharp20 and/or x11-toolkits/gtk-sharp30.

Remember, mcs is the unified C♯ compiler. No need to run gmcs nor dmcs.